Treatment of leather



Patented Jan. 11, 1938 PATENT OFFICE 2,105,446 TBEATIWENT OF LEATHER John Arthur Wilson, Milwaukee, Wis., assignor,

by mesne assignments, to Monsanto Chemical Company, a corporation of Delaware No Drawing. Application April 1, 1935, Serial No. 14,125

V 5 Claims.

The present invention relates to a process of treating chrome-tanned leather and the products obtained thereby. It is more particularly directed to a process wherein there is incorporated into the customary pretreating or neutralizing bath prior to dyeing and fatliquoring an alkali metal polyphosphate.

It is well known to those skilled in the art that after. the customary tanning operation the leather is usually subjected to a pretreatment bath containing borax preparatory to dyeing and fatliquoring. Other salts of weak acids have also been used, but borax has generally been favored. v

vAn objection to a material like borax is the difficulty of hydrogen ion control in the subsequent fatliquoring treatment, a treatment which is most efiective when carried out within the range of pI-I value from 4.0 to 4.5 for the used fat-liquor. When borax is used in the pretreating baths, the pH variation is rather wide. An excess of borax causes the leather to become too soft and flabby. When an alkali metal polyphosphate is employed according to my inven- Pickling is done by immersing the skins in a bath of salt and sulfuric acid and allowing them to remain in the bath with occasional agitation until equilibrium has been established.

The pickled skins are then tanned in the usual chrome bath using, for example, 8 pounds of Tanolin-R dissolved in l2 gallons of water per 1001poundsof pickledfweight. Other chrome tanning liquors may also be used. The tanning operation is carried out in the usual manner, after which the stock is set out, shaved and weighed. 2

I now subject the'hides to a pretreatment prior to dyeing and fatliquoring. The pretreatment consists in immersing the tanned and shaved skins in a solution of an alkali metal polyphosphate, or more particularly sodium metaphos phate.

The stock is placed in the pretr'eating drum and washed with running water at 70 F. for an hour,

after which the water is drained off. This serves asa preliminary wash. The skins in the drum are again covered with water and the drum rotated. At this point 10 pounds of sodium metaphosphate in 50 gallons of water is slowly added to the drum, taking 20 minutes to add. After the solution is all in, the drum is rotated for 5 minutes longer, the solution drained and the skins washed in running water for 20 minutes.

In place of the sodium metaphosphate employed above, I may generally use an alkalimetal metaphosphate, although because of cheapness and availability the sodium salt is to be preferred. I may also employ together with the sodium metaphosphate other salts and especially other orthophosphates and pyrophosphates. The two pyrophosphates which may be employed are the acid alkali-metal pyrophosphate and the neutral alkali-metal pyrophosphate, or the socalled tetra pyrophosphate. Indeed, I may employ such a pyrophosphate to the exclusion of the metaphosphate at this point in the tanning process, as I have found generally, that an improved product is obtained when employing those sodium phosphates which may be designated as molecularly dehydrated phosphates. By molecularly dehydrated phosphates I mean the salts of such phosphoric acids as are formed upon the removal of water of constitution from phosphoric acid.

Of particular value for the present process are those molecularly dehydrated phosphates which may be considered as the stoichiometrically equivalent mixtures of metaphosphates and tetra pyrophosphates, even though they can not be considered the chemical equivalents of such mixtures. Such phosphates are obtained by heating and fusing mixtures of monobasic and dibasic alkali-metal orthophosphates or by mixing and heating together certain alkali-metal salts and phosphoric acid. The heating is preferably car-' ried to the point of complete fusion of the mass, at which point the water of constitution has been eliminated and a polyphosphate formed. Satisfactory products for the present purpose may be made as described in the British Patent 378,345, of 1932, due regard being had to the choice of -a product having a major proportion of sodium metaphosphate equivalent for the best results in tained when utilizinga composition having a pH ranging downwardly from, say, 7.0 to 4.5 and I therefore prefer to use polyphosphates which develop such hydrogen ion concentration before coming into contact with the leather in. the treating bath, which, depending upon the acid in the leather, produces in the treating bath a pH value to about 3.5 to 4.5.

After the leather stock has been thoroughly penetrated by the polyphosphate solution, the same is drained oil and the skins are washed in running water, drained, and then dyed.

The dyeing operation may be carried out in the usual manner as well understood in the art, and may be followed by a topping off with basic dyes, which treatment may sometimes be desirable when employing my process, as I have found that the phosphate causes a deeper penetration of dyestufl. v

j After the dyeing operation I add the fatliquor to the hides. A suitable composition for fatliquor consists, per 1000 pounds of hides, of 1.6 pounds fig soap, 4 pounds sulfonated neatsfoot oil, and 12 pounds of chamois moellon degras in 50 gallons of water at F. This operation prevents excessive shrinking of the leather and gluing together of the fibers upon drying out, and finishing, which is carried out in the regular way.

Some of the advantages which I have discovered as resulting from the pretreatment with the phosphate salts are shown in the table below. For the purpose of comparison I am also giving the results obtained by the older method which, up to the time of the present invention, represented the best method of treatment.

Table Water ab- Rela- H sorption Neutraliz ing agen: fire: 'l ensgil t a tiva in re rea men e s on porosbani ity so 24 mins. hrs.

1% sodium metap osphate 13.26 4,613 1,776 4.35 41.7 83.0 114% borax 12.78 4,617 1,554 4.!) 24.2 56.0

' take up the perspirationmore readily, and pass it on to the outer surface from which it evaporates. The nature of finishing the leather is such as to make the grain surface water repellant, andso no complication is involved through making the leather take up more water from the outside.

Tanners are finding it advantageous to use a smaller amount of fatliquor of a more acid nature. I have found, for example, that the best range for the used fatliquor should be between a pH of 4.0 and 4.5. In a series of tests in which the metaphosphate content of the pretreatment bath was varied between 0.5% and 2.0%, the pH of the used fatliquor showed values of 4.35, 4.43, 4.43, and 4.21. The values obtained with the same amount of borax in each case (l gave used fatliquors having a pH of 4.80, 5.15, 5.14 and 4.35, respectively. The ease of pH 'control when using my invention is therefore obvious.

It is my belief that the molecularly dehydrated phosphate employed in the pretreatment of leather as above outlined combines directly with the leather, forming an insoluble complex therewith. This is indicated by the behavior of the leather on the subsequent dyeing and fatliquoring operations, in spite of the fact that a final wash with water is employed at the conclusion of the pretreatment operation, which presumably would wash out most soluble compounds. What the compound is, I am at present unable to state; however, there seems to be ample evidence for its existence.

While I have described my invention in one form, it will be appreciated that it is susceptible to various changes and modifications without departing from the spirit thereof, and I desire, therefore, it be not limited. except as indicated by the prior art or as particularly pointed out in the claims. I

What I claim is:

1. The improvement in treating leather comprising subjecting hides after tanning but before fatliquoring to the action of a solution of an alkali-metal polyphosphate formed by the reaction of an alkali metal metaphosphate and an alkali metal pyrophosphate.

2. The improvement in treating leather comprising subjecting hides after tanning but prior to dyeing and fatliquoring to the action of a dilute solution of sodium metaphosphate.

3. The process of treating hides comprising pickling and chrome tanning the hides in the usual way, subjecting the hides to a pretreatment bath comprising a sodium metaphosphate in dilute solution, and then fatliquoring the hides.

4. The process of treating hides for the production' of leather comprising chrome tanning the hides in the usual wayand then subjecting the hides to a pretreatment bath comprising a solution of sodium metaphosphate, then dyeing and fatliquoring the hides.

5. Chrome tanned leather containing combined phosphate and possessing a relative porosity in the neighborhood of 1700 and a 30-minute water absorption value in the neighborhood of 40.

JOHN ARTHUR wr'son. 

